The endometrium is the lining of the uterus where the embryo implants and grows into a baby. This is what it should look like just before you ovulate (the egg is released).

polyp within the endometrium


This picture shows a uterus with a polyp within the endometrium. It is clearly seen as a white lump between the two crosses. As you can imagine, this would prevent the embryo from implanting properly in the uterus.